In the ongoing quest to decrease the impact buildings have on the environment, we have begun to hear more about the concept of net-zero-energy buildings. The term itself sounds impressive. After all, when looking at low-energy use, how can you beat zero? But what exactly does that mean? How difficult is it to achieve? Is there one definition of net-zeroenergy buildings, or does it depend on your point of view? Paul Torcellini, team leader for commercial buildings research at the National Renewable Energy Laboratory, Golden, Colo., has spent a number of years looking at all different sides of the net-zero proposition. He researches and develops methods that properly integrate energy features into building design. Recently, eco-structure had the opportunity to speak with Torcellini about the considerations, challenges and possibilities of zero.

Why do you think there is growing attention to the idea of net-zero-energy buildings?

PT: I think there’s a real appeal to the vision that buildings can get to the point where they’re no longer a burden on the environment or infrastructure. Can buildings become producers rather than consumers? From a purely sustainable point of view, buildings always will be consumers. They’re made of materials; they take up a land resource. Although they’ll always consume, at some point they can give something back to achieve sustainability.

What does net-zero energy mean?

You can ask 100 different people and get 100 different definitions. We’ve always talked about zero-energy buildings because when you say something like “a 30 percent reduction in energy performance” one end of the scale must be standard practice or code and the other end must be zero by default. The implication is that a 100 percent reduction would be zero cost or zero energy.

When we really started concentrating on this a few years ago, many questions came up and we asked whether this was achievable. Maybe we only can get to within 20 percent of zero, or maybe buildings can all export energy and be less than zero. There are many considerations when defining the term. Does the net-zero equation include transportation energy? Do you have food in the building? Where does that food come from and how was it produced? We have restricted our definitions to the energy in and out of a building. You have to draw the line somewhere because otherwise it becomes so complex that it unravels and no progress will be made. We standardized four definitions of zeroenergy buildings. The first, and probably the most common, is a net-zero-site-energy building. The boundary is the site, and the energy is measured annually at the utility meters. By default, the point of purchase of the energy defines the site boundary. The next definition is a net-zero-source-energy building. In this definition, the energy is valued at its point of extraction—the wellhead or coalmine. Because we really cannot measure this, site to source conversions are available to take site metering and convert it to source energy.

Another definition is net-zero-energy cost. This is where the credits received on energy exported equal the amount of energy bills. Last is the net-zero-energy-emission building. In this definition, the emissions factors, which are derived from the site-to-source calculation, are shown to be zero. Within the definitions, we also can talk about grades of zero-energy buildings. The highest grade would be a building that’s completely autonomous from the grid and produces as much energy annually as it uses. It’s not connected to the grid in any way, and the owner doesn’t bring in any energy from outside. This is very difficult to achieve. Even current grid-independent houses tend to use propane as a backup and cooking source. Equal in grade is a building that is connected to the grid but sends energy to the grid and never takes from the grid. Also a zero-energy building but not as “good” is a building that buys and sells energy to the grid. Typically the building pushes energy to the grid during the day and then buys it back at night. You can have the discussion that you’re probably buying coal electrons at night and displacing natural-gas electrons during the day, depending on the utility dispatch. These dispatch issues get tricky. Last would be a building that buys green offsets and meets the zero-energy-source definition. Again, you’re dealing with how energy is dispatched and that someone has the ability to sell renewable electrons and can account for them on an annual basis. Philosophical discussions arise about this because we ask whether an energy-inefficient building should be able to say it has solved its problems by buying offsets.

How did NREL come to these definitions?

We did an assessment study looking at buildings across the U.S., asking what kinds of buildings can get to zero, what kinds of characteristics they have and what percentage in energy savings do we need to achieve so that the roof area could produce the rest of the energy with photovoltaics. Certain buildings are easier than others in the commercial sector. For example, warehouses have fairly low loads and lots of roof area. High-rise buildings have a much tougher time. We quantified the fact that the probability of getting to net-zero energy diminishes significantly after about 2 stories. One of the things we considered was that the roof area of the building really is the only footprint it owns. You can plan to cover the field next to the building with PVs to get to zero, which is a reasonable path until the value of the real estate goes up and somebody constructs a building there and you lose your PV. We strived to document definitions and issues and benefits to the definitions to help understand issues about boundaries and metrics.

So the boundary isn’t as simple as saying that a building that is off the grid is net-zero energy?

I have yet to see someone that has made a gridindependent, net-zero-energy building. Almost inevitably, even with residential, people still will use propane for cooking, clothes drying or other high-intensity things. Maybe someone has met all the loads and produces his or her own liquid fuels or uses wood from the site for cooking, but it certainly is not mainstream, even in the zeroenergy- building world. As an example, we just put in a wood-chip burner at NREL, and our estimate is that 80 percent of our heating for the NREL campus will come from wood chips. That’s great because we’re offsetting a huge amount of natural gas, but that’s not completely renewable because something has to chip that wood and transport it to our site. We figure roughly 3 percent of that number is diesel. That’s not bad, but it’s not zero. One of the benefits of zero is that it is a hard number; you meet the definition or you don’t. We’re still in a situation where it is a lot cheaper to conserve energy than produce it. There are some great tools and technology out there. We have condensing boilers, higher-efficiency air conditioners, better fan technology and methods to better integrate these. There is a big potential for integration of parts for more energy efficiency, but there is a limit, and supply-side options need to be explored.

How much impact does regional climate have on how you design a net-zero-energy building?

If you consider the PVs on the roof, some of the hotter climates are the easiest to get to net zero because of sun intensity and the ability to use low-energy cooling solutions. As you move farther north, it gets more difficult. From a strictly energy point of view, heating requires a lot of energy. In the southern U.S., people talk about their air-conditioning bills. But on a really hot day, you might have a 20 F [11 C] temperature differential from inside to outside and, depending where you are, you might have to remove humidity. But on a cold day in Chicago, you could have a 70 F [39 C] difference from inside to outside. The heating side dominates the energy problem.

What are some considerations for designers, architects or engineers setting out to design a net-zero-energy building?

If you want to design a net-zero-energy building, you really have to plan for 20 percent beyond zero. Buildings never perform as well as you think they’ll perform. The PV never generates as much as expected and there always are things people sneak into buildings that end up using more energy. It’s very important to remember the details, like smoke alarms, for example. My house has seven smoke alarms and, of course, that’s a good thing. The other side of it is that if you start adding those up across the country, you find that an entire power plant's produced energy is used to power smoke alarms. Think about what happens to the building at night. A typical office building is occupied five or maybe six days per week for maybe 10 to 12 hours. Remove holidays. When you add it together, the building is unoccupied approximately 70 percent of the year. What we’ve found when we’ve measured buildings is that they don’t turn off very well. Think of all those electronics with their blinking lights in homes and offices running all the time. If you want to get to zero, you have to start looking around at everything that’s plugged in. You need to look at things like server management, information-technology structure and uninterruptible power supplies. Then you can get into things like paint and carpet colors and how they impact daylighting. It’s integrated design. We can make a huge difference in getting to zero by understanding that every design decision has an energy impact. Remember that if you’re committed to making a net-zero-energy building, whatever energy you save is energy you don’t have to produce.

For more information, visit www.eere.energy.gov/buildings/highperformance/zero_energy_buildings.html.